An aqueous solution of sodium chlorate and sodium chloride is conventionally produced by the electrolysis of aqueous sodium chloride in diaphragmless electrolytic cells. The extent of electrolysis is controlled to produce an effluent from the cell in which the sodium chlorate and sodium chloride have the desired ratio, usually in the range of about 1:1 to about 20:1 and preferably in the range of about 2:1 to about 15:1. The aqueous solution may be further processed to crystallize out the sodium chlorate for sale in crystal form for a variety of purposes, for example, in the production of chlorine dioxide for use in the bleaching of chemical cellulosic pulps, by reduction in the presence of a strong mineral acid, usually sulphuric acid, or aqueous solution may be used directly for that purpose.
In the electrolysis of sodium chloride to form sodium chlorate, it is conventional to add chromates, usually in the form of sodium dichromate, to the electrolyte in the cell to improve significantly the current efficiency of the cells in the conversion of sodium chloride to sodium chlorate. The cell effluent, also known as "cell liquor", therefore, generally contains significant amounts of chromate ion.
It is desirable to remove chromate ion from the cell effluent before employment of the same in chlorine dioxide generation and it is desirable to recover the chromate ion for reuse in the electrolytic cells. In addition, chromate ions are a toxic pollutant, so that environmental considerations require removal of the chromate ions where discharge of an effluent stream containing such ions may be effected. A number of prior proposals have been made for the removal of chromate ion from cell liquor.
U.S. Pat. No. 3,843,769 teaches a process whereby alkali metal chlorate solutions containing hexavalent chromium are treated with at least a 3-molar proportion of a water-soluble sulfide, such as sodium sulfide, sodium bisulfide, potassium bisulfide or hydrogen sulfide. The solution then is acidified to a pH below about 5, whereby the trivalent chromium is precipitated as an insoluble product, and removed from the solution. In addition to producing trivalent chromium precipitate, elemental sulfur is also precipitated. Further, the cell liquor contains dissolved sulphide ions, which is detrimental in chlorine dioxide production, since the acid medium would tend to form H.sub.2 S from the sulfide ions, which then would react violently with the already-unstable chlorine dioxide. In addition, the pH adjustments required to be effected consume large quantities of chemicals and require large capital expenditures.
The problem of elemental sulfur coprecipitation is overcome by the proposal in U.S. Pat. No. 4,268,486 to use hydrazine to reduce Cr.sup.VI to Cr.sup.III but the problems of undesirable reaction with ClO.sub.2 and pH adjustment remain. An additional drawback of this process is that, if the reaction is not carried out quickly enough at the correct pH, then the hydrazine or hydrazine salt is consumed by reaction with the chlorate instead of the Cr.sup.VI. Hydrazine is also suggested as a suitable reducing agent in Canadian Patent No. 1,139,080. A similar process, described in German O.S. 3,032,131 discloses hydroxylamine hydrochloride as a reductant but again the same deficiencies exist.
U.S. Pat. No. 4,259,297 describes a process for the removal of hexavalent chromium from alkali metal chlorate solutions by reaction with, inter alia, alkali metal and alkaline earth metal sulfites, bisulfites and dithionites. In this process, an initial pH adjustment to the range of about 9 to about 13 is effected, the reductant is added to form an aqueous slurry of solid particles of trivalent and divalent chromium compounds, the pH of the slurry is adjusted to a value of about 2 to about 4, the pH of the slurry is again adjusted to a value of about 6 to about 8, and the solid particles are removed from the aqueous solution.
The use of low pH's in the range of about 2 to about 4, as required in this prior art, may lead to chlorine dioxide formation from the chlorate cell liquor, which is very hazardous. In addition, no method is provided for the removal of excess reductant and losses of sodium chlorate occur.
In U.S. Pat. No. 4,704,289, assigned to the assignee hereof and the disclosure of which is incorporated herein by reference, there is described an effective chemical procedure for the removal of hexavalent chromium from cell liquor in which Cr.sup.(III) is formed or precipitated as Cr(OH).sub.3 by reduction of hexavalent chromium under alkaline conditions with a dithionite, under certain specific process conditions.